The cerebellum, consisting of 80% of the neurons in the human brain, is involved in balance and motor coordination, and also modulates language, reasoning and social processes via its forebrain circuits. The developing cerebellum is particularly sensitive to factors that impact on growth (or cause injury) around birth, since much of its growth occurs in the third trimester and continues after birth. The postnatal cerebellar cortex has two proliferating stem/progenitor populations, one dedicated to making excitatory granule cells and the other to interneurons and astrocytes. Whereas great advances have been made in defining the stem/progenitor cells and lineages that generate the developing cerebellum, little is known about the ability of the cerebellum to produce new cells following injury. We recently discovered that the mouse cerebellum has a large capacity to replenish cells killed around birth. First, we found that Nestin-expressing progenitors (NEPs) that normally are dedicated to the astrocyte lineage are reprogrammed to become granule cell precursors (GCPs) when the latter are killed by irradiation or genetic approaches. Furthermore, at least two spatially and transcriptionally distinct NEP subtypes that are lineage-restricted have different responses to the loss of GCPs to achieve proper scaling of cell types after injury. Second, Purkinje cells (PCs), which are born by embryonic day 13.5, are rapidly replaced via proliferation of rare immature Purkinje cells (iPCs) following PC killing, and replenishment of PCs is age-dependent. Finally, signals released by dying cells, such as reactive oxygen species (ROS), and cells in the microenvironment can have critical influences on repair responses of progenitor cells. Preliminary results showed that cells in microenvironment have distinct cellular responses in each of our injury models. We will address two critical questions for both injuries: i) What are the gene expression changes that underlie the cellular transitions necessary for cell replenishment and ii) what are the roles of dying cells, immune cells and glia in regeneration. Our central hypothesis is that cerebellar progenitors and rare immature neurons maintain distinct transcriptional plasticity and along with cells in the microenvironment respond differently to killing of GCPs and PCs. Our specific aims are to: 1) Uncover the transcriptional signatures of NEP subtypes and iPCs during development and regeneration, and identify pathways required for proliferation and neuron production using single cell and mutant analyses. 2) Determine how the microenvironment influences NEP and iPC injury responses.